Autodetect the max zoom level based on the highest resolution coverage

[emxsys]

Overriding findMaxZoomAuto to return the zoom level closest to the highest resolution coverage in the request.

• Re: issue #75

[emxsys]

@pdavidc You're review is welcome, but not required to move forward.

[emxsys]

Problem

When a GeoPackage-based layer is exported as a GeoPackage, the max zoom level is determined to be the max zoom level in the gridset (level 21), not the max level in the tileset.

This is problematic: it creates a request for a GeoPackage at the highest possible resolution, many levels greater than required, causing a seemingly never-end job that consumes large amounts of memory and disk resources.

The coverage object that provides the gridset is unaware of the actual number of levels in the tileset.

Options/Ideas

• Possibly trim the gridset. This would have to be performed at the time the data store is created as the grid range is stored in the data store's XML.
• Possibly interrogate the data store. Metadata? Metadata would have to be added to the coverage/data store instances.
• Create a UI for exporting to a GeoPackage that doesn't depend on the ability to autodetect the max zoom level in this case, but instead relies on the user to select a max zoom level (with constraint).

[emxsys]

@pdavidc I would appreciate a sanity check on the changes while I move into functional testing.